The present invention relates to light emissive devices and, in particular, to light emissive devices having an improved contrast ratio.
Flat panel display technologies offer various advantages over conventional cathode ray tubes, such as a greatly reduced physical profile, lower power and voltage requirements, a reduced heat output, and lighter weight. For the next generation of flat panel displays, advances are being sought to improve display contrast, image brightness, efficiency, color purity, resolution, scalability, and reliability while also reducing the costs of fabrication. Light emissive flat panel displays generally include a layered structure of thin and thick films formed on a transparent front electrode and a transparent substrate. The layered structure is patterned as a rectangular array of active elements or pixels which are arranged in multiple rows and columns. To form an image, individual pixels radiate visible light when lit and are nominally dark when in an unlit state. The display contrast is quantified as the ratio between the brightness of a typical pixel when lit and the brightness of reflected light when that typical pixel is unlit. In standard operating environments, ambient light from sources such as sunlight and artificial room lighting is reflected by the unlit pixel and the surrounding area of the transparent panel. Pixel blooming is another phenomenon that degrades display contrast in which light from a lit pixel propagates through the plane of the transparent substrate and is emitted as visible light by adjacent pixels which may be in a nominally unlit state. To generate high contrast images, various conventional contrast-enhancement techniques are utilized either to reduce reflection of light by the flat panel display or to reduce pixel bloom.
One method of enhancing the contrast of a light emissive flat panel display is to apply a contrast-enhancement layer to the front transparent surface or glass panel of the display. Certain contrast-enhancement layers function as a polarizing filter to reduce the light reflected from the display by as much as 75 percent. Although polarizing filters improve the display contrast in most lighting conditions, they are costly and reduce the display brightness by more than 50 percent. Other contrast-enhancement layers function as anti-reflective films that eliminate the reflection of light at the front glass/air interface. However, anti-reflective films are only viable for those flat panel displays that include a non-reflective opaque layer within the layered structure that absorbs incident light. Often, the non-reflective opaque layer must be added to the layered structure as an additional layer. For enhancing the contrast of flat panel displays, polarizing filters and anti-reflective films have a significant limitation in that neither type of contrast-enhancement layer can alleviate pixel blooming. In fact, polarizing filters worsen pixel blooming.
Another method of enhancing the contrast of a light emissive flat panel display having a fully-transparent or semi-transparent display structure is to apply a light-absorbing layer on the non-light emitting side of the layered structure. The light-absorbing layer absorbs residual light from external sources transmitted through the transparent display structure without outward reflection by the various layer interfaces. However, a transparent conductor must be added to the layer structure to cover the light-emitting layer. This addition of the transparent conductor significantly increases the fabrication cost. Moreover, the light-absorbing layer is only viable for small-area displays because the lower conductivity of the requisite transparent conductor layer prevents the formation of long length electrodes.
Some conventional contrast-enhancement techniques tailor layers of the layered structure of thin and thick films forming the light emissive flat panel display to serve as contrast-enhancement layers. Tailoring of the existing essential layers eliminates the need for additional fabrication steps that add special purpose light-absorbing layers to the layered structure. For example, one contrast-enhancement technique for plasma displays provides the device electrodes with a black conductive layer formed from a paste of inert metal particles, such as silver or gold, and a black colorant additive. However, the black conductive layer is formed in front of the light emitting layer in the layered structure and, as a result, is not applicable for high-resolution displays in which the use of the black conductive layer would reduce the pixel aperture. Another contrast-enhancement technique applicable for thin film electroluminescent displays is to darken or blacken the rear metal electrode by fabricating it from a layer that has a composition graded from aluminum oxide to aluminum. However, the contrast enhancement afforded by such a blackened rear metal electrode is limited because the interfaces of the transparent layers overlying the blackened rear metal electrode still provide a significant reflection of the incident ambient light. Yet another contrast-enhancement technique applicable for thin film electroluminescent displays and direct current organic light emitting diode displays is to interpose a multilayer optical interference layer among the layers of the flat panel display that enhances the absorption of ambient light. In yet another contrast-enhancement technique, an insulating material, such as a plastic or an oxidized metal, is applied as a low-reflectance surface treatment in the lateral spaces separating adjacent pixels.
There is a need for a contrast-enhancement technique that is generally applicable to flat panel display technologies and which can be incorporated into the layered structure of the device with few additional fabrication steps or little added structure and without degrading the display properties of the flat panel display due to its incorporation.
The present invention is premised on the realization that with proper selection of thick film dielectric material and colorant, an electrically insulating dielectric layer, typically a thick film dielectric layer, found in most alternating current electroluminescent displays and alternating current plasma displays can be endowed with light-absorbing properties. According to the present invention, a dielectric layer, such as a thick film dielectric layer, can be rendered black for absorbing a broad range of the visible spectrum or, in the alternative, can be rendered colored to reflect only a specific range of wavelengths within the visible spectrum. The colorant, such as black pigment particles of a black ink additive, color pigment particles of a color ink additive, or active particles of a dye, does not significantly modify the dielectric constant of the layer so that it retains a high dielectric constant when used to pass alternating current.
According to an aspect of the present invention applicable in other light emissive flat panel display technologies, a color or black dielectric layer can be employed as an insulating layer/protective film on the rear surface of the display screen. The color or black dielectric layer is formed from a proper selection of a thick film dielectric material having a porous or polycrystalline microstructure and a colorant, such as active particles of an organic dye, black pigment particles of a black ink additive, or color pigment particles of a color ink additive. The presence of the colorant significantly reduces the reflectivity of the grain boundaries of the polycrystalline microstructure dielectric material or the pore walls of the porous microstructure dielectric material.
According to the present invention, the color or black dielectric layer is applied by standard techniques that produce the desired electrical and structural properties followed by coloring or blackening through the application of the colorant. As a result, little additional processing is required to introduce the color or black dielectric layer of the present invention into the fabrication of the layered structure of conventional flat panel displays. Furthermore, the present invention advantageously affords independent control of the electrical, structural, and optical properties of the dielectric layer. The electrical performance of the color or black dielectric layer of the present invention is comparable to other high performance black dielectric materials, such as plastics and oxidized metals, yet absorbs a much larger percentage of the incident ambient light than heretofore achievable with such standard high performance dielectrics. The black dielectric layer of the present invention provides high contrast when used in an electroluminescent flat panel display due to the darkening of the background about the pixels. The color dielectric of the present invention provides a vivid display appearance when used in an electroluminescent flat panel display, such as providing a dark blue background for a yellow light emission by a pixel.
The dielectric layers of the present invention have a thickness in a range between about 5 xcexcm to about 100 xcexcm and have colorant dispersed at grain boundaries or on the internal surfaces of pores throughout the thickness thereof. Thick film dielectric layers retain their electrically insulating character and are not susceptible to premature electrical breakdown, due to uniformity irregularities, which would cause a thin film dielectric layer to experience breakdown. Furthermore, thick film dielectric layers may be advantageously applied by screen printing, which is a simple, high yield, and easily scalable process.
The objects and advantages of the present invention will be further appreciated in light of the following detailed description and drawings in which: